Previously, the following systems have been known to the prior art:
1. Using a single-beam system the light of a series of incandescent projection lamps is directed to a series of coordinated photo-receivers after the light has traversed the protection zone. The photo-receivers are all illuminated and are therefore conductive only if none of the light beams is interrupted. The disadvantage of this system is that the dark conductivity of one or more photo-receivers cannot be readily detected and consequently the affected photo-receivers signal the bright state even if the light beam is interrupted and do not result in the machine being shut down.
2. A further prior art system utilizes an incandescent projection lamp, an optical system, a semi-transparent reflector, a rotary reflector and a parabolic reflector. The rotary reflector is situated at the focal point of the parabolic reflector and light emitted by the lamp is projected at different angles of incidence on to the parabolic reflector, thus producing parallel motion of the light beam. The pencil of light strikes a reflector, is reflected into itself, is coupled on to the photo-receiver through a semi-transparent reflector and produces a photoelectric current. Entry into the protected zone results in a brief interruption of the pencil of light which moves parallel to itself. Such a pulse may be adapted to shut down the machine.
Disadvantages of the system are due to the need for using a relatively expensive optical system and the limited adaptation facilities.
3. A further prior art system utilizes a fluorescent tube instead of incandescent projection lamps, the light of the said fluorescent tube being projected via individual optical systems on to appropriately coordinated photo-receivers so that alternating voltage signals are produced by virtue of the alternating voltage feed of the fluorescent lamp. These signals are able to stop the machine.
A disadvantage of this system is its sensitivity to external light since fluorescent lamps are also used for illuminating working rooms.
4. Gallium-arsenide diodes, which have since been developed for these and other purposes may also be used in photoelectric guards in place of projection lamps and fluorescent tubes, digital processing of the pulses being performed according to a prior art technique by an AND-network or a multi-vibrator logic system which replaces such a circuit. Relatively substantial proneness to trouble is a disadvantage of this system.